<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>
  <script>
    class Father {
      constructor(x, y) {
        this.x = x;
        this.y = y;
      }
      sum() {
        console.log(this.x + this.y);
      }
    }

    class Son extends Father {
      constructor(x, y) {
        super(x, y)
        this.x = x;
        this.y = y;
      }
      jian(x, y) {
        console.log(this.x - this.y);
      }
    }

    const son = new Son(5,3);
    son.jian()

  </script>
</body>
</html>